Lacalli EvoDevo 2012, 3:12 http://www.evodevojournal.com/content/3/1/12

COMMENTARY Open Access The Middle Cambrian fossil Pikaia and the evolution of chordate swimming Thurston Lacalli*

Abstract Conway Morris and Caron (2012) have recently published an account of virtually all the available information on Pikaia gracilens, a well-known Cambrian fossil and supposed basal chordate, and propose on this basis some new ideas about Pikaia’s anatomy and evolutionary significance. Chief among its chordate-like features are the putative myomeres, a regular series of vertical bands that extends the length of the body. These differ from the myomeres of living chordates in that boundaries between them (the myosepta) are gently curved, with minimal overlap, whereas amphioxus and vertebrates have strongly overlapping V- and W-shaped myomeres. The implication, on biomechanical grounds, is that myomeres in Pikaia exerted much less tension on the myosepta, so the animal would have been incapable of swimming as rapidly as living chordates operating in the fast-twitch mode used for escape and attack. Pikaia either lacked the fast-twitch fibers necessary for such speeds, having instead only slow-twitch fibers, or it had an ancestral fiber type with functional capabilities more like modern slow fibers than fast ones. The first option is supported by the sequence of development in zebrafish, where both myoseptum formation and fast fiber deployment show a dependence on slow fibers, which develop first. For Pikaia, the absence of fast fibers has both behavioral and anatomical implications, which are discussed. Among the latter is the possibility that a notochord may not have been needed as a primary stiffening device if other structures (for example, the dorsal organ) could perform that role. Keywords: Pikaia, Basal chordates, Myomere and notochord evolution, Amphioxus, Swimming mechanics, Vertebrate muscle fibers, Yunnanozoans

Background muscles, and the authors identify an axial trace that Zoology texts typically list four diagnostic features of could be either a notochord or a notochord and nerve chordates: pharyngeal (that is, gill) slits or pores, a noto- cord combined. In addition, however, there are peculiar chord, a dorsal nerve cord and serial (or segmental) features not known from living chordates: a sausage- muscles. This last feature, represented by the somite- shaped dorsal organ running the length of the trunk, derived myomere series in the case of cephalochordates and an anterior shield-like structure, the anterior dorsal (amphioxus) and vertebrates, is the subject of this ac- unit, covering the head region. Pikaia does not, there- count, stimulated by the recent description [1] by Simon fore, fit entirely comfortably with modern chordates, Conway Morris and Jean-Bernard Caron (here referred suggesting that it is either divergent, if it is a chordate, to as CMC) of the Middle Cambrian fossil Pikaia graci- or is a basal member of the chordate lineage differing in lens (Figure 1) from the Burgess Shale of British significant ways from surviving members of that lineage. Columbia. The authors interpret Pikaia as a basal Of the chordate features listed above, the first, the chordate and, though this conclusion is provisional, it pharyngeal slits, have an evolutionary history that prob- would be perverse to deny the key similarities between ably predates chordates by a considerable interval, be- this animal and what would be expected of a basal cause apparently homologous structures occur in more chordate: much of the body is occupied by a series of basal deuterostome phyla, in living hemichordates and vertical bands resembling the septa between segmental fossil echinoderms [2-4]. Pharyngeal pores or slits, where they occur, are assumed to play an ancestral role in Correspondence: [email protected] deposit- or filter-feeding as a means for disposing of ex- Biology Department, University of Victoria, Cunningham Building, Victoria, cess water entering the mouth and pharynx with food BC V8W-3N5, Canada

© 2012 Lacalli; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Lacalli EvoDevo 2012, 3:12 Page 2 of 6 http://www.evodevojournal.com/content/3/1/12

overlap between adjacent myomeres nowhere exceeds more than about one segment. CMC discuss this in relation to the much more angled V- and W-shaped myomeres of amphioxus and vertebrates, but there is more to be drawn from this comparison in my view. Tunicate larvae are omit- ted from both CMC and this account because, based on current phylogenies [6], both they and their tail muscula- ture are reduced secondarily to a degree that makes them uninformative regardingthenatureofancestral myomeres. Vertebrates, as one would expect, have the most compli- cated segmental architecture. Myomeres in adult fishes are substantial masses of tissue whose thickness seriously com- plicates the task of analyzing the internal stresses generated during swimming. This is chiefly because the tension Figure 1 Pikaia gracilens, as reconstructed by Conway Morris and Caron [1].The head bears a pair of tentacles, probably sensory exerted on the spinal axis as a given fiber contracts varies in nature, and paired rows of ventrolateral projections that may be with the distance that fiber lies from the central axis. To gills. Not shown: the expanded anterior (pharyngeal) region of the avoid producing unequal stresses across the myomere, the digestive tract, and the dorsal shield-like structure, the anterior fibers are arranged helically and the myosepta have a com- dorsal unit, that lies above it. The boxed detail shows the main axial plex three-dimensional shape approximating a W [7,8]. The features: the dorsal organ (do), and the putative notochord (not) and digestive tract (dt). The size range among specimens is 1.5 to accepted explanation, which requires some fairly sophisti- 6 cm, which makes this animal very close in size to the adult stage cated modeling [9], is that this arrangement minimizes the of modern lancelets (amphioxus). difference in tension experienced on the opposing faces of each myoseptum as a means of reducing energy loss. Myomeres in larval fish and in amphioxus are chevron- particles. The remaining three features mentioned above, or V-shaped and are much less massive. They are flattened which together constitute the axial complex, are respon- mediolaterally and so, in effect, can be treated as a first ap- sible for undulatory swimming. These are considered to proximation as flat, two-dimensional structures. This be restricted to chordates alone, and form a suite of makes it much easier to understand the biomechanical characters that are closely linked anatomically, and that issues involved without resorting to a detailed mathemat- operate functionally as a unit [4]. Of the three, only the ical and computer analysis. Muscle fibers in amphioxus at- notochord has a plausible counterpart, the enteropneust tach to the myosepta directly [10] rather than to the stomochord, in more basal deuterostomes, but the two notochord sheath, so the greatest stresses on the noto- structures are not currently considered to be homolo- chord are borne where the septa join the sheath. But the gous [5]. We thus have very little evidence from com- whole complex is also bound tightly together by sheets of parative studies to indicate how the axial complex basal lamina, and therefore acts as a single unit, as evolved, all at once or step by step, and if the latter, what opposed to relying on an array of subsidiary tendons and the key steps might have been. CMC have made a start, bony struts to distribute the forces, as in fish. So, using in their paper, by discussing the role the dorsal organ amphioxus as a model, what are the advantages of having may play in antagonizing the force of muscle contraction V-shaped myosepta rather than vertical ones? The key during swimming, at a stage in chordate evolution when seems to be in the angle of inclination, that is, the steep- the notochord might not yet have fully taken over this ness of the V. Because the muscle fibers are aligned hori- function. This commentary on their paper is an attempt zontally, along the body axis, it is in that direction that to carry the argument somewhat further, using simple force is exerted during contraction. Expressed as vectors biomechanical principles to clarify the relation between (Figure 2A), this force has two components, one directed myomere morphology and swimming mode, and to dis- perpendicular to the anterior surface of the myomere (P cuss some of the implications this has for the behavior in the figure), the other parallel to it (along the dashed and mode of life of Pikaia and its kin. line). The former is the more important component, as stresses in that direction are the ones most likely to result Myomere shape and its implications in mechanical failure of the septa, for example, by tearing CMC discuss myomere shape at some length because of the myomeres apart, whereas forces acting parallel to the significant ways this differs between Pikaia and modern septa would only be important if damage by shear were chordates. Myomeres in Pikaia are taller and narrow (box, the key issue. From simple trigonometry, the perpendicu- Figure 1), while the myosepta, which form the interface be- lar component of force is less than the total force of con- tween them, are gently curved, or sigmoidal, such that the traction in proportion to the cosine of the angle the Lacalli EvoDevo 2012, 3:12 Page 3 of 6 http://www.evodevojournal.com/content/3/1/12

septa attaching to it (Figure 2B). This evens out the stress due to contraction. In amphioxus, the overlap ranges from one to two segments in larvae to three to four in the adult, considerably more in the latter than in Pikaia for an animal of approximately similar size.

Multiple locomotory modes and escape from predators The implication of the steeper angle of incline of amphi- oxus and vertebrate myosepta, in comparison with Pikaia, is that the forces generated by myomere contraction are a Figure 2 The beneficial consequences of having chevron- or more serious issue for amphioxus and fish, requiring this V-shaped myomeres, illustrated using amphioxus. (A) The V adaptation, than for Pikaia.Itisthusreasonabletosuppose shape guarantees that the force acting perpendicular to the myoseptum (vector P) is less than the force of contraction (vector F) that Pikaia myomeres would not have been capable of by an amount that increases with increasing angle (Θ) to the exerting as much force as those of modern chordates, and vertical. The degree of incline shown is typical for amphioxus larvae, consequently peak swimming speed in Pikaia would have and increases with increasing age. (B) Somite overlap in young adult been considerably less. There is an assumption here, as the amphioxus, modified from [11]. The central components of the conclusion depends on the supposition that angling of locomotory system are the notochord (not, shown in violet) and the nerve cord (green). These are bound to the myomeres (pink) by myosepta gives a direct measure of the force of contraction sheaths of basal lamina (blue). The V-shaped myomeres are across a range of taxa, both living and long extinct. For this positioned so that the tip of the caudal-most in any section is to be true, the angle of inclination must be a comparatively adjacent to the notochord, while the extended tails of progressively plastic feature in evolutionary terms, one that is able to more anterior myomeres (shown in progressively lighter shades, adapt rapidly in any lineage when it is advantageous to do compare with A) are ranged above and below. Because every point along the notochord has essentially the same complement of septa, so. This may be reasonable because, as structural features this arrangement ensures that the force of contraction experienced go, myomere shape is one that in principle should be easy by the notochord is distributed evenly along its anteroposterior axis, to adjust incrementally. Indeed, myomeres in nearly con- rather than being borne at specific sites. temporaneous vertebrates such as Haikouichthys [12], which are roughly intermediate in shape between those of myoseptum makes with the vertical: for given force of Pikaia and amphioxus, show an incremental gradation in contraction, increasing this angle (Θ in the figure) shape along the body. Those in the caudal region are more decreases the stresses on the myoseptum. For amphioxus, steeply inclined, which is precisely where one would predict from my own material and published photos [11], the the greatest stress if, for example, the animals had an escape measured incline is in the range of 40 to 55° in larvae, de- response involving powerful tail flips. pending on stage, and 64 to 68° for adults. This translates There is a further issue that needs to be considered in into a reduction of force by roughly 24 to 43% in larvae this context, and that is the nature of the muscle fibers and 56 to 62% in adults. This is a significant benefit if, in themselves. Myomeres in amphioxus and fish have basically consequence, less is required by way of reinforcement to two fiber types with quite different functional capabilities the enclosing system of sheath and septa. [13,14]. Rapid, episodic burst swimming results from the There is another advantage in arranging for the boundar- contraction of fast (or fast-twitch, or deep) fibers, of which ies between myomeres to be steeply inclined: it maximizes the bulk of the myotome is composed. In addition, how- the overlap between myomeres and so distributes the force ever, there are sets of slow (slow-twitch, or superficial) of contraction more evenly along the length of the noto- fibers along the lateral margin of the myomere, fibers that chord. To illustrate this, consider an alternative, of straight do not fatigue as quickly and are responsible for prolonged myosepta transversely oriented more or less vertically, as in bouts of slower swimming. These are used for long distance Pikaia. If the muscles in such an animal attach only at the swimming (for example, cruising or migration) in fishes, for septa (as they do in amphioxus), all the force of contraction diurnal vertical migrations in the water column in amphi- would be borne at the sites where these join the notochord oxus larvae, and to adjust position in burrows in adult sheath. Along the sheath, therefore, sites of maximum and amphioxus. With regard to amphioxus specifically, there is minimum stress would alternate, so there is both a greater a lingering misconception that the burrowing habit has risk of catastrophic failure (for example, by kinking) and resulted in its becoming a rather poor and ineffective swim- less likelihood that the structure as a whole will flex mer. The contrary is, in fact, the case, as anyone who has smoothly. In contrast, when septa are inclined to allow for tried to net an amphioxus swimming at peak speed will at- multiple overlaps, every point along the notochord bears test. The animal is mass of muscle and connective tissue essentially the same load as measured by the number of capable of powerful writhing movements when restrained, Lacalli EvoDevo 2012, 3:12 Page 4 of 6 http://www.evodevojournal.com/content/3/1/12

and of considerable speed when swimming, greater than are significant differences in the way the two systems are many fish when adjusted for relative body size [15]. In part innervated. From data on larvae [17,26] fast fibers are this is possible because, lacking eyes, amphioxus need not innervated by neurons distributed along the nerve cord look where it is going. Instead, and because of its tough cu- via conventional synapses, and mechanosensory input is ticle, it simply bounces off obstacles in its path. Such beha- routed to this system alone as befits an escape pathway. viors are due to the action of the fast fibers, and are well The slow system, in contrast, is innervated by a dedicated described in previous accounts of the behavior of larvae series of neurons in the anterior nerve cord via a less spe- [16,17] and adults [18]. Clearly, a much greater physical cialized type of synaptic contact. The principle inputs, so stress is being exerted on the support structures of the body far as this has been determined, is from neurons in the during fast than slow swimming, which must certainly be a cerebral vesicle, specifically in the amphioxus homolog of factor in explaining the adaptive advantages of V-shaped the hypothalamus, and from the dorsal ocelli (eyespots). myotomes. In both the hypothalamic and ocellar pathways, paracrine The conclusion one can draw from this is, that because release from large terminals predominates over specialized Pikaia lacks steeply inclined myosepta, its muscle fibers synapses. The former, where it occurs (for example, in have properties more like slow fibers than fast ones. If it core limbic elements of the vertebrate brain), is generally could be shown that the slow system evolved first, Pikaia considered an indication of evolutionary antiquity [27]. could quite logically be interpreted as representing a stage The involvement of the ocelli in the slow circuits may also in chordate evolution before the evolution of fast fibers. be significant, since response to light both during vertical With some further biomechanical analysis of contraction migration (in larvae) and in burrows (adults) are aspects strengths, septum thickness, and so on, a good deal could of feeding behavior, and hence sufficiently basic to survival probably be inferred about the behavioral capabilities and that they probably predate the rise of visual predators. In mode of life of Pikaia based on the known properties of addition, because the neurotransmitter in these photore- modern slow fibers. However, the evidence for a sequence ceptors is related to a gonadotropin-releasing hormone in the evolution of fiber type, of slow before fast or vice [28], there is a possibility that the ancestral function of the versa, is at best circumstantial. Logically, one might suppose slow system was in some way involved with reproduction that the less effective locomotory mode would have evolved or mating behavior. first, with subsequent improvements being a response to While the above is consistent with the slow system extraordinary new adaptive pressures. Since the Lower to being evolutionarily older, there is an alternative sug- Middle Cambrian was a period when fast-swimming preda- gested by molecular data on fiber type specification: that tors with high-resolution eyes were appearing (chiefly the the fast fibers are the default state of myocyte develop- anomalocarids and their kin [19]), slow modes of swim- ment, while the slow fibers follow a divergent pathway ming that would have been adequate for basal chordates initiated by early Hedgehog signaling from the noto- nearer the dawn of the Cambrian would have been increas- chord [29]. This would imply that slow fibers, at least in ingly less so. Faster modes would have had to evolve. That their modern form, are a late addition to an older pro- this happened quickly is evident from the vertebrates of the gram of myocyte differentiation. However, even if the time, for example, Haikouichthys, as mentioned above, mechanism controlling fast fiber differentiation is an an- which predates Pikaia by 5 million years at least. cient one, it does not mean that the ancestral fibers Conodonts, though much later, also have this feature matched modern fast fibers in their functional cap- [20,21], as befits another putative basal vertebrate. abilities. Quite the converse, since on the evidence Both belong to lineages that survived the Cambrian, of myomere shape in Pikaia, it would seem that the whereas Pikaia evidently, so far as we know, did not. forces generated by the ancestral fibers were much The developmental sequence is suggestive here as well. less, probably more like modern slow fibers than fast In fish (from zebrafish data), the slow fibers develop first, ones. Suppositions concerning the behavior and and then migrate to the outer surface of the somite mode of life of Pikaia need then to be assessed with [22,23]. The fast fibers develop later, but depend on cues this in mind. from the slow fibers for correct deployment [24], while myoseptum formation is also impaired in the absence of General anatomy and mode of life: some slow fibers [25]. That slow fibers are so fundamentally im- conjectures portant to the normal sequence of developmental events If one assumes that Pikaia was incapable of a fast escape is certainly consistent with an early origin, possibly predat- from visual predators of the Anomalocaris type, a strategy ing the evolution of the later developing fast fiber system, of avoidance would likely be its preferred solution. Pikaia but there are other interpretations (see below). Further was probably not, therefore, an animal that spent a lot of evidence for the relative antiquity of the slow system, time high up in the water column or near the surface dur- again circumstantial, comes from amphioxus, where there ing daylight hours. One could envisage it feeding Lacalli EvoDevo 2012, 3:12 Page 5 of 6 http://www.evodevojournal.com/content/3/1/12

inconspicuously near the bottom, perhaps cruising along played this role either alone or in concert with a noto- and browsing on benthic detritus or microbial mats, direc- chord. The dorsal organ is the relevant axial structure ted by its paired sensory tentacles. This accords with the here, as it may have been some kind of turgid sac or rod. conclusions of CMC that, from the mode of preservation, Viewed from the perspective of modern chordates, few Pikaia was likely epibenthic rather than fully pelagic. Al- zoologists would accept, given its extreme dorsal loca- ternatively, it may have migrated vertically to spend time tion, that such a structure would be an effective device feeding at the surface, but only at night. Unfortunately, as for attaching myomeres and antagonizing their contrac- the apparently tiny mouth is difficult to interpret in the tion. As CMC point out, however, and discuss at some fossils, there is little direct evidence from the morphology length, a truly basal chordate might well have had a as to how Pikaia fed. Because the mouth was not large, it provisional and less efficient support system prior to the seems unlikely that Pikaia took in large volumes of water evolving one dependent on the notochord alone. The when feeding, however, as a suspension feeder normally dorsal organ could have been adequate for this purpose. would be expected to do. Myomeres would attach along the sides of the dorsal The series of paired appendages projecting from the organ and extend ventrally from it, probably attached to pharyngeal region may be gills, and there are indications a midsagittal sheet (or sheets) of connective tissue. in some specimens of small pores at their bases. The pres- Waves of contraction passing along the body would have ence of projecting external gills rather than slits would least amplitude at the level of the dorsal organ, and be- imply that the oxygen demand of even slow swimming come progressively more pronounced ventrally as dis- required them. This is perhaps surprising given the flat- tance from the dorsal organ increased. The undulations tened cross-sectional profile of the body, as it has consid- produced would thus be graded along the dorsoventral erable surface area for respiration across the skin alone. axis, from shallow dorsally to deeper ventrally. A noto- This is evidently sufficient for amphioxus, where gas ex- chord could add accessory stiffening at roughly the level change is predominantly across the general surface of the of the gut, which may explain why it derives embryo- body [30]. A need to expand the gas exchange surface fur- logically from the same rudiment as the latter. Indeed, ther in Pikaia could be an indication that it experienced as locomotory capabilities of early chordates improved, severe anoxia, at least periodically, due to its habitat. there might have been an increasing need for such a stif- There is a further problem in the putative gills being fening device to damp undulations that would otherwise located so far from the bulk of the trunk musculature. An impair the movement of the products of digestion along extensive vascular system would then be needed to dis- the digestive tract. A notochord that evolved initially for tribute oxygenated blood to the trunk, so vascular traces this purpose would be well placed to later take on a should be expected in suitably preserved material. CMC more central role in body support as the need arose. in fact identify one such trace, of what may be a ventral As CMC also point out, having the dorsal organ act in blood vessel, but this begs the question of why there are this fashion links Pikaia to another group of puzzling fos- no indications of vessels needed to complete the circuit. sils of Cambrian age, the yunnanozoans. These are consid- On comparative anatomical grounds, these should lie ered by many to be chordates or close to them [31,32], above the alimentary canal in a situation comparable to but there are other possibilities [33-35]. Yunnanozoans the chordate dorsal aorta. have a dorsal sail-like structure that is apparently cuticu- The notochord and nerve cord, expected as parallel larized and at least partially articulated. Muscle fibers have axial traces, pose a different problem, because there is been reported in association with this structure [31], no a priori reason to suppose that they should have the though this interpretation is contentious, but the very fact same close linkage with the muscles as in modern chor- of articulation implies an ability to flex due to the action dates. Undulatory swimming is accomplished without a of underlying muscles. Assuming this is a valid interpret- notochord in diverse taxa, ranging from small (fluke cer- ation, yunnanozoans provide a second example of axial cariae) to large (segmented annelids, for example, Nereis, support in a putative chordate by a dorsal structure that is some species of which attain lengths of more than a not a notochord. With a second structure available to meter). In chordates, the notochord acts as a compres- share the load, the ancestral notochord would have been sion strut, resisting the force of contraction during freed of at least some of the constraints that determine its swimming. The need for this would be especially acute size, robustness and location in modern forms. The fossils during rapid escape swimming, but much less so for an need to be interpreted with this in mind. animal capable only of slow swimming. This is not to say that Pikaia would not have had a notochord (though Conclusions this cannot be ruled out), only that it need not have been The nearly vertical junctions between myomeres in the the core structure around which the myomeres are orga- putative basal chordate Pikaia gracilens are shown to be nized. Any number of other axial structures might have indicative of a muscle fiber type more similar to the slow Lacalli EvoDevo 2012, 3:12 Page 6 of 6 http://www.evodevojournal.com/content/3/1/12

fibers of modern chordates than fast fibers. It is not 17. Lacalli TC, Kelly SJ: Ventral neurons in the anterior nerve cord of known if the former evolved first, but if so, Pikaia may amphioxus larvae. I. An inventory of cell types and synaptic patterns. J Morph 2003, 257:190–211. represent a stage in evolution before chordates had fast 18. Webb JE: The role of the notochord in forward and reverse swimming fiber systems. If not, then Pikaia was nevertheless prob- and burrowing in the amphioxus Branchiostoma lanceolatum. J Zool Lond ably limited to a relatively slow form of locomotion. This 1973, 170:325–338. 19. Paterson JR, Garcia-Bellido DC, Lee MSY, Brock GA, Jago JB, Edgecombe GD: has implications for the mode of life, implying passive Acute vision in the giant Cambrian predator Anomalocaris and the origin avoidance rather than fast escape from predators, and for of compound eyes. Nature 2011, 480:237–240. the anatomy, in that less stiffening of the body is needed 20. Aldridge RJ, Briggs DEG, Smith MP, Clarkson ENK, Clark NDL: The anatomy of conodonts. Phil Trans R Soc Lond B 1993, 340:405–421. to oppose the contractions of the axial musculature. This 21. Pridmore PA, Barwick RE, Nicoll RS: Soft anatomy and the affinities of last point could explain the prominence of the dorsal conodonts. Lethaia 1996, 29:317–328. organ in Pikaia, as an alternative support structure at a 22. Devoto SH, Melancon E, Eisen JS, Westerfield M: Identification of separate slow and fast muscle precursor cells in vivo, prior to somite formation. time when the notochord played a more limited role in Devel 1996, 122:3371–3380. locomotion. 23. Ingham PW, Kim HR: Hedgehog signalling and the specification of muscle cell identity in the zebrafish embryo. Ex Cell Res 2005, 306:336– Competing interests 342. There are no competing interests. 24. Henry CA, Amacher SL: Zebrafish slow muscle migration induces a wave of fast muscle morphogenesis. Dev Cell 2004, 7:917–923. Acknowledgments 25. Henry CA, McNulty IM, Durst WA, Munchel SE, Amacher SL: Interactions Supported by the Natural Sciences and Engineering Research Council of between muscle fibers and segment boundaries in zebrafish. Dev Biol Canada (NSERC Canada). I thank Nick Holland and Jon Mallatt for helpful 2005, 287:346–360. comments on the manuscript, and Stephen Devoto, Jean-Bernard Caron and 26. Lacalli TC: The dorsal compartment locomotory control system in Dale Stokes for additional ideas and information. amphioxus larvae. J Morph 2002, 252:227–237. 27. Nieuwenhuys R: Comparative aspects of volume transmission, with a Received: 3 April 2012 Accepted: 13 June 2012 sidelight on other forms of intercellular communication. Prog Brain Res Published: 13 June 2012 2000, 125:49–126. 28. Castro A, Becerra M, Manso MJ, Sherwood NM, Anadon R: Anantomy of the References Hesse photoreceptor cell axonal system in the central nervous system of 1. Conway Morris S, Caron J-B: Pikaia gracilens Walcott, a stem-group amphioxus. J Comp Neurol 2006, 494:54–62. chordate from the Middle Cambrian of British Columbia. Biol Rev 2012, 29. Roy S, Wolff C, Ingham PW: The u-boot mutation identifies a 87:480–512. Hedgehog-regulated myogenic switch for fiber-type diversification 2. Swalla BJ, Smith AB: Deciphering deuterstome phylogeny: molecular, in the zebrafish embryo. Genes Dev 2001, 15:1563–1576. morphological and palaeontological perspectives. Phil Trans R Soc Lond B 30. Schmitz A, Gemmel M, Perry SF: Morphometric partitioning of respiratory 2008, 363:1557–1568. surfaces in amphioxus (Branchiostoma lanceolatum). J Exp Biol 2000, 3. Sato A, Rickards B, Holland PWH: The origin of graptolites and other 203:3381–3390. pterobranchs: a journey from ‘Polyzoa’. Lethaia 2008, 41:303–316. 31. Mallatt J, Chen J-Y: Fossil sister group of craniates: predicted and 4. Gutmann WF: The hydraulic principle. Amer Zool 1988, 28:257–266. found. JMorph2003, 258:1–31. 5. Ruppert EE: Key characteristics uniting hemichordates and chordates: 32. Chen J-Y: Early crest animals and the insight they provide into the homologies or homoplasies? Can J Zool 2005, 83:8–23. evolutionary origin of craniates. Genesis 2008, 46:623–639. 6. Delsuc F, Tsagkogeorga G, Lartillot N, Philippe H: Additional molecular 33. Shu DG, Conway Morris S, Zhang ZF, Han J: The earliest history of the support for the new chordate phylogeny. Genesis 2008, 46:592–604. deuterostomes: the importance of the Chengjiang Fossil-Lagerstätte. 7. Alexander RM: Principles of Animal Locomotion. Princeton: Princeton Proc R Soc Lond B 2010, 277:165–174. University Press; 2003. 34. Bergström J: In The earliest arthropods and other animals,InDarwin’s 8. Shadwick RE, Gemballa S: In Structure, kinematics, and muscle dynamics in Heritage Today. Edited by Long M, Zhou Z, Gu H. Beijing: Higher Education undulatory swimming,InFish Physiology. Vol. 23. Fish Biomechanics. Edited by Press; 2010:42–29. Shadwick RE, Lauder GV. San Diego: Academic; 2005:241–280. 35. Donoghue PCJ, Purnell MA: Distinguishing heat from light in debates 9. Van Leeuwen JL: A mechanical analysis of myotome shape in fish. J Exp over controversial fossils. Bioessays 2009, 31:176–189. Biol 1999, 202:3405–3414. 10. Gemballa S, Weitbrecht GW, Sanchez-Villagra MR: The myosepta in doi:10.1186/2041-9139-3-12 Branchiostoma lanceolatum (Cephalochordata): 3D reconstruction and Cite this article as: Lacalli: The Middle Cambrian fossil Pikaia and the microanatomy. Zoomorphology 2003, 122:169–179. evolution of chordate swimming. EvoDevo 2012 3:12. 11. Ruppert EE: In Cephalochordata (Acrania),InMicroscopic Anatomy of Invertebrates. Vol 15. Edited by Harrison RW, Ruppert EE. New York: Wiley-Liss Inc; 1997:349–504. 12. Shu D-G, Luo H-L, Conway Morris S, Zhang X-L, Hu S-X, Chen L, Han J, Zhu M, Chen L-Z: Lower Cambrian vertebrates from South China. Nature 1999, Submit your next manuscript to BioMed Central 402:42–46. and take full advantage of: 13. Bone Q: Evolutionary patterns of axial muscle systems in some invertebrates and fish. Amer Zool 1989, 29:5–18. • Convenient online submission 14. Lacalli TC, Kelly SJ: Somatic motoneurons in amphioxus larvae: cell types, • Thorough peer review cell position and innervation patterns. Acta Zool 1999, 80:113–124. 15. Webb JE: In A review of swimming in amphioxus,InPerspectives in • No space constraints or color figure charges Experimental Biology. Vol 1. Zoology. Edited by Davis PS. Oxford: Pergamon • Immediate publication on acceptance Press; 1976:447–454. 16. Stokes MD: Larval locomotion of the lancelet Branchiostoma floridae. J • Inclusion in PubMed, CAS, Scopus and Google Scholar Exp Biol 1997, 200:1661–1680. • Research which is freely available for redistribution

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